Essentially it is known that an thermal magnetic circuit breaker is a manually or automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Therefore the thermal magnetic circuit breaker has for example at least one magnetic trip device in order to prevent the electrical circuit or an electrical device from damage by short circuit and a thermal trip device in order to prevent the electric circuit or an electrical device from damage by overload. A short circuit is an abnormal connection between two nodes of the electric circuit intended to be at different voltages. And especially in reference to a molded-case circuit breaker, a short-circuit is an abnormal connection between two separate phases, which are intended to be isolated or insulated from each other. This results in an excessive electric current, named an overcurrent limited only by the Thévenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. An overload is a less extreme condition but a longer-term over-current condition as a short circuit.
The magnetic trip device has at least an armature element moveable arranged with respect to a yoke or especially a current conduction element conducting electrical energy or current, respectively. The armature element or armature, respectively, is a magnetic element and especially a pole piece having at least partially an iron material and reacting to a magnetic field created by the yoke during a trip moment. In order to realize a guided movement of the armature element towards the yoke at least during a trip event like a short circuit, the armature element is arranged on an armature locator. The armature locator is moveable arranged on a pin extending from an adjustment bar towards the yoke. The armature locator or the adjustment bar can be connected with a trip bar which is able to interrupt a current flow of the current circuit, when the trip bar is moved due to a movement of the armature locator or the adjustment bar in conjunction with the armature element towards the yoke because of a magnetic force.
Disadvantageously, the armature locator oscillates on an axis of the pin during a presence of high current and therefore during the trip event is occurred. This oscillation can cause the armature locator to be in an angled or inclined position, which increases friction during movement and affecting the response time during the trip event. To minimize this behaviour, the length of a contact area of the armature locator around the pin needs to be sufficient. However, having a common adjustment of more than one armature locator requires utilization of a common adjustment bar, which limits available space and restricts size of this contact area.
Furthermore, a calibration spring arranged between the armature or the armature locator, respectively, and the yoke requires a minimum space to reach a solid height. In the magnetic trip unit the working position of this calibration spring and the required forces at those positions define the calibration spring dimensions. That means that the solid spring height resulting from the calibration spring design is a restriction that must be taken into account, because armature locator movement could be stopped when the spring reaches its solid state and is therefore totally compressed.